The purpose of the proposed study is to collect basic data on the intraspecific aggressive behavior (agonistic behavior) of female housemice (mus musculus). Ebert (1972) found that, after rearing in isolation, 25% of a population of wild-trapped female mice attacked a C57BL female intruder. Artificial selection for this female agonistic behavior was begun and is currently at generation S6. The proposal requests funding for a continuation of the selection program and ten related experiments. The relationship between reproductive fitness and agonistic behavior for females will be investigated. Maternal effects on aggressive behavior will be investigated using cross-fostering. Maternal aggression, non-isolation-induced aggression, male aggression, and intersex aggression will all be investigated as correlated characters. The development of female aggression will be studied. The last set of four experiments will investigate physiological influences by looking at estrous cycle fluctuations in agonistic behavior, endogenous endocrine characteristics of the selection lines (plasma testosterone levels and testosterone uptake in the brain), and the effects of ovariectomy, adrenalectomy, and replacement injections of testosterone and estrogen. The final experiment will investigate pheromonal influences on female aggression.